(a) Field of the Invention
This invention relates to a process for conducting exploration of the earth's sub-surface up to depths of about fifty feet, by the use of electromagnetic waves emanating from an airborne source.
(b) Prior Art
Various forms of wave energy have been used for the purpose of "mapping" differing depths of the sub-surface of the earth without using equipment which physically penetrates the surface. Although some such techniques have been attempted commercially, there is not now a commercially usable process for relatively high speed and relatively accurate "mapping" of the sub-surface of the earth.
For example, when pipelines are to be installed, in areas such as Alaska, knowledge of the condition of the earth's crust is important in order to know where patches of ice or other obstacles can be found. It is desirable to know this in advance, before digging a trench to bury the pipe, in order to avoid substantial expenditures of time and money, digging in areas which have substantial patches of ice or other obstacles, because of the expense and difficulty of digging through ice or boulders, for example.
After a pipeline has been laid below the surface in a frozen area, such as in Alaska, it is important to check the condition of the ground surrounding the pipe line to locate any areas in which the ice has thawed, because if substantial thawing takes place, the ground support of the pipe section in the thawed area is lost, and the pipe might collapse, resulting in leakage and loss of the oil or gas transported in the pipeline and the considerable expenditure required to locate, excavate and repair the leaking pipeline and the attendant environmental damage.
One well known pipeline is the oil pipeline which transports crude oil in Alaska from Prudhoe Bay to Valdez. Another pipeline is planned to transport natural gas from Prudhoe Bay, Alaska to Chicago and to San Francisco. These pipelines are of enormous importance to the energy independence and economy of the United States, and the extreme weather conditions present in Alaska (and other similar geographic areas) makes it important to be able to do sub-surface surveying accurately and speedily and with minimum cost. The expense of digging and tunnelling makes it desirable to know the condition of the sub-surface in areas of less extreme climactic conditions, such as where large underground boulders might present a problem.
There are two prior art techniques which are now used commercially for checking for the presence or absence of ice below the earth's surface. The first involves digging a bore hole, typically about fifty feet deep, and taking a core sample (e.g., fifty feet deep by four inches in diameter) to look at an actual physical cross-section of the soil to determine its condition. This technique is obviously slow, and it takes approximately fifteen hours to dig a single hole. As of mid-1980, the cost of digging a single bore hole in Alaska was about $15,000.00. Typically, these bore holes are dug at intervals of about one mile, because of their considerable expense. The bore hole technique will give an accurate indication of the condition of the sample taken, but it does not necessarily give an accurate indication of sub-surface conditions proximate to or remote from the bore hole. For example, the bore hole could be two or three feet from a sub-surface ice formation, and yet not detect the presence of that ice.
Another technique which is currently commercially used involves the measurement of the resistivity of a given sub-surface area, in which probes are inserted into the ground to measure the resistivity of the area beneath and around the probes. This technique, which had an approximate cost of $1,300 per mile as of mid-1980, will provide relevant information for a cube, each side of which is twelve feet by twelve feet and centered about the probe, but this technique is also slow, does not provide all of the necessary information, and is limited to the area measured.
Numerous articles have been published and patents granted which deal with equipment and techniques to locate sub-surface ice formations, but none of those which are known is satisfactory for use on a commercial scale, because they do have the capability to get a sufficient depth or a sufficient accuracy or resolution of the area being surveyed. For example, U.S. Pat. No. 3,665,466 shows one technique for measuring the thickness of ice, but this process is limited to the ability to measure sea ice at the surface.
To the best of applicant's knowledge, there is no presently available or disclosed technique which has the capability of measuring cross-sections of the sub-surface of the earth to depths of fifty feet at a relatively high speed of, for example, fifty to one hundred miles per hour.
The importance of accurately laying out a path for an underground pipe line is indicated by the fact that the cost of making a trench in Alaska, which is set up as an assembly line operation, was approximately $1,500,000 per day at mid-1980. Therefore, if a trench is improperly laid out and must be re-routed, substantial waste will have occurred.
FIG. 1 is a chart which compares certain significant aspects of the two prior art techniques for geotechnic surveying with the instant invention.